Strongly Luminous Tetranuclear Gold(I) Complexes Supported by Tetraphosphine Ligands, meso- or rac-Bis[(diphenylphosphinomethyl)phenylphosphino]methane

A series of tetragold(I) complexes supported by tetraphosphine ligands, meso- and rac-bis[(diphenylphosphinomethyl)phenylphosphino] methane (meso- and rac-dpmppm) were synthesized and characterized to show that the tetranuclear Au-I alignment varies depending on syn- and anti-arrangements of the two dpmppm ligands with respect to the metal chain. The structures of syn-[Au-4(meso-dpmppm)(2)X]X-3' (X=Cl; X'=Cl (4a), PF6 (4b), BF4 (4c)) and syn-[Au-4(meso-dpmppm)(2)]X-4 (X=PF6 (4d), BF4 (4e), TfO (4 f); TfO=triflate) involved a bent tetragold(I) core with a counter anion X incorporated into the bent pocket. Complexes anti-[Au-4(meso-dpmppm)(2)]X-4 (X=PF6 (5d), BF4 (5e), TfO (5 f)) contain a linearly ordered Au-4 string and complexes syn-[Au-4(rac-dpmppm)(2)X-2]X-2' (X=Cl, X'=Cl (6a), PF6 (6b), BF4 (6c)) and syn-[Au-4(rac-dpmppm)(2)]X-4 (X=PF6 (6d), BF4 (6e), TfO (6 f)) consist of a zigzag tetragold(I) chain supported by the two syn-arranged rac-dpmppm ligands. Complexes 4d-f, 5d-f, and 6d-f with non-coordinative large anions are strongly luminescent in the solid state (lambda(max)=475-515 nm, Phi=0.67-0.85) and in acetonitrile (lambda(max)=491-520 nm, Phi=0.33-0.97); the emission was assigned to phosphorescence from 3[d(sigma*sigma*sigma*)p(sigma sigma sigma)] excited state of the Au-4 centers on the basis of DFT calculations as well as the long lifetime (a few mu s). The emission energy is predominantly determined by the HOMO and LUMO characters of the Au-4 centers, which depend on the bent (4), linear (5), and zigzag (6) alignments. The strong emissions in acetonitrile were quenched by chloride anions through simultaneous dynamic and static quenching processes, in which static binding of chloride ions to the Au-4 excited species should be the most effective. The present study demonstrates that the structures of linear tetranuclear gold(I) chains can be modified by utilizing the stereoisomeric tetraphosphines, meso- and rac-dpmppm, which may lead to fine tuning of the strongly luminescent properties intrinsic to the Au-4(I) cluster centers.